MWException: Log stack traces for php errors (not exceptions)

* Remove use of 'error' where it's redundant.
* Remove call to logException from responsibility of MWException.
  Call from exception handler instead.

Change-Id: I8764cf5df87b226813c9b9cf99f9b4f3fa4b7c92
(cherry picked from commit 399ba2fecfe098b45b2064380eadcac0b24b9a4a)